Field of the Invention
The present invention relates to an image forming apparatus that scans a scan-target surface by deflecting a luminous flux using a rotating polygonal mirror.
Description of the Related Art
An optical scanning apparatus that is used in an image forming apparatus such as a laser printer scans a photosensitive member by deflecting a luminous flux from a light source using a rotating polygonal mirror of an optical deflector, in order to form a static latent image on the photosensitive member. However, there are cases in which the reflection surfaces of the rotating polygonal mirror have a portion that is not parallel to the rotation axis, i.e., cases in which a so-called plane tilt has occurred, depending on the cutting accuracy at the time the rotating polygonal mirror was manufactured and the attachment accuracy at the time the rotating polygonal mirror was attached to the optical deflector. If deflection scanning is performed with a luminous flux in a situation where a plane tilt has occurred, a phenomenon occurs in which the scanning position of the luminous flux is constantly displaced from the target position. Also, in some cases, the reflection surfaces each have a different curve, depending on the cutting accuracy of each reflection surface. If deflection scanning is performed with a luminous flux in this situation, so-called jitter occurs in the main scanning direction, which is a phenomenon in which the scanning position of the luminous flux on each reflection surface is constantly displaced from the target position. Although it is possible to solve these phenomena by increasing the cutting accuracy of the rotating polygonal mirror, this solution increases the processing cost.
For this reason, Japanese Patent Laid-Open No. 2007-286129 discloses a configuration in which the reflection surfaces of the rotating polygonal mirror are specified and variations in the scanning line interval are electrically corrected. According to Japanese Patent Laid-Open No. 2007-286129, a magnet is attached to a lower portion of a reflection surface of the rotating polygonal mirror, and the reflection surface is specified using a Hall element that is located below the rotating polygonal mirror. Also, Japanese Patent Laid-Open No. 2007-078723 discloses a configuration in which a reflection surface of the rotating polygonal mirror is specified based on the phase relationship between a synchronization signal used for writing start position control and a rotation angle signal used for rotation control of the rotating polygonal mirror.
According to the configuration disclosed in Japanese Patent Laid-Open No. 2007-286129, it is necessary to provide a magnet, a Hall element, and so on for the rotating polygonal mirror, and the cost is high. According to the configuration disclosed in Japanese Patent Laid-Open No. 2007-078723, it is not possible to uniquely specify the reflection surfaces until the number of synchronization signals and the number of rotation angle signals that are detected during one rotation of the rotating polygonal mirror are relatively prime.